Relative yield of charged and neutral heavy meson Pairs in e ⁺e ^- annihilation near threshold

The subject of the charged-to-neutral yield ratio for BB̄ and DD̄ pairs near their respective thresholds in e ⁺e ^- annihilation is revisited. As previously argued for the B mesons, this ratio should exhibit a substantial variation across the Υ(4S) resonance due to interference of the resonance scattering phase with the Coulomb interaction between the charged mesons. A simple alternative derivation of the expression describing this effect is presented here, and the analysis is extended to include the D-meson production in the region of the ψ(3770) resonance. The available data on kaon production at the φ(1020) resonance are also discussed in connection with the expected variation of the charged-to-neutral yield ratio.